Prepare for upcoming change to HttpRequest and HttpClientResponse (#12)

* Prepare for upcoming change to HttpRequest and HttpClientResponse

An upcoming change to the Dart SDK will change `HttpRequest` and
`HttpClientResponse` from implementing `Stream<List<int>>` to
implementing `Stream<Uint8List>`.

This forwards-compatible change prepares for that SDK breaking
change by casting the Stream to `List<int>` before transforming
it.

dart-lang/sdk#36900

* drop +1

Author: tvolkert

lib/event_source.dart

@@ -132,6 +132,7 @@ class EventSource {
     // is only called once per connection.
     var reconnectOnce = _onceFunc(_reconnect);
     response
+        .cast<List<int>>()
         .transform(utf8.decoder)
         .transform(LineSplitter())
         .listen(_onMessage, onDone: reconnectOnce, onError: (_) {